1. Field of the Invention
This invention relates to means of stimulating electrically excitable tissue, and more particularly relates to means for adjusting the locus at which action potentials are induced in such tissue.
2. Description of the Related Art
Two major practical problems reduce the efficacy of epidural spinal cord stimulation (SCS) for pain control. One is the difficulty of directing the stimulation-induced paresthesia to the desired body part and the other is the problem of disagreeable sensations or motor responses to the stimulation, which reduce the comfortable amplitude range of the stimulation. It is generally agreed that in SCS, for chronic pain, paresthesia should cover the whole pain region. With present stimulation methods and equipment, only highly skilled and experienced practitioners are able to position a stimulation lead in such a way that the desired overlap is reached and desired results are obtained over time with minimal side effects. It requires much time and effort to focus the stimulation on the desired body region during surgery and, with single channel approaches, it is difficult to redirect it afterwards, even though some readjustments can be made by selecting a different contact combination, pulse rate, pulse width or voltage.
Redirecting paresthesia after surgery is highly desirable. Even if paresthesia covers the pain area perfectly during surgery, the required paresthesia pattern often changes later due to lead migration, or histological changes (such as the growth of connective tissue around the stimulation electrode) or disease progression. The problem of lead placement has been addressed by U.S. Pat. No. 5,121,754 by the use of a lead with a deformable distal shape. These problems are not only found with SCS, but also with peripheral nerve stimulation (PNS), depth brain stimulation (DBS), cortical stimulation and also muscle or cardiac stimulation.
A system capable of some adjustment of spinal cord excitation is described in PCT International Publication No. WO 95/19804. However, that system requires three electrodes, optimally spaced, which is a serious handicap during the surgical procedure required in order to place these electrodes in the body. Three electrodes may require the use of a paddle arrangement which is surgically difficult to manipulate adjacent the spinal cord. In addition, that system has only limited adjustment capability, dependent on the distance from the electrodes to the spinal cord.